Is there a rule on acronyms being spelled out with the first letter of each word being uppercase or lowercase?
- interim final rule (IFR)
- Interim Final Rule (IFR)
The Chicago Manual of Style, sixteenth edition (2010) briefly addresses the question of whether the spelled-out form of an initialism or acronym should be initial-capped if the short form is capitalized, at 10.6 Capital versus lowercase for acronyms and initialisms:
On the other hand, if the words in a spelled-out version of an acronym or initialism are not derived from proper nouns or do not themselves constitute a proper noun (as in the official name of an organization), they should generally be lowercased, even when they appear alongside the abbreviated form.
[Example:] transmission-controlled protocol/Internet protocol (TCP/IP)
This is an accurate representation of standard U.S. publishing-house practice, in my experience, but there are lots of instances where at least some publishers seem to be influenced by the capitalization of the initialism to initial-cap the spelled-out form as well. Thus, for example, the first 50 matches that a Google search returns for the phrase "sports utility vehicle (SUV)," where the phrase appears in the course of a normal sentence (and not in, say, a title or subhead), include 38 instances of "sports utility vehicle (SUV)" and 12 instances of "Sports Utility Vehicle (SUV)."
Contrarily, the first 50 matches that a Google search returns for the phrase "interim final rule (IFR)," where the phrase appears in the course of a normal sentence, include 20 instances of "interim final rule (IFR)" and 30 instances of "Interim Final Rule (IFR)"—although in this case a few of the initial-capped matches may be parts of longer proper names, as in this sentence from the website Human Rights Watch:
Human Rights Watch unequivocally opposes the Interim Final Rule (IFR) on Moral Exemptions and Accommodations for the Coverage of Certain Preventative Services under the ACA (CMS-9925-IFC).
In any event, if "interim final rule (IFR)" is not part of a proper name, I see no reason to buck what Chicago calls the general convention to render the spelled-out form of the term in all-lowercase letters. But if you prefer to initial-cap the spelled-out term, you can find many examples online where other writers and publishers have chosen to do exactly that.
No, there is no such rule.
It depends entirely on what you're abbreviating. The International Monetary Fund is the proper name of an organization, so if you spell out the abbreviation IMF, you capitalize.
On the other hand, scuba stands for self-contained underwater breathing apparatus. That's not a brand name or anything, so it's not a proper noun and not capitalized.
Or, if it concerns you that scuba is also lowercase, try CSMA, which stands for carrier-sense multiple access.
Initialisms are typically capitalized to avoid confusion with short words or abbreviations. This is the case regardless of whether the constituents it represents would be capitalized or not, and regardless of whether the abbreviation has been lexicalized (i.e. come to be seen as a word in and of itself).